[PATCH] tree-optimization/100112 - VN last_vuse and redundant store elimination

[Richard Biener via Gcc-patches]

This avoids the last_vuse optimization hindering redundant store
elimination by always also recording the original VUSE that was
in effect on the load.

Bootstrapped and tested on x86_64-unknown-linux-gnu.

I'm still pondering on how to avoid the wastage of adding the ref
twice and will at least record some statistics for this.

2021-09-27  Richard Biener  <rguent...@suse.de>

        PR tree-optimization/100112
        * tree-ssa-sccvn.c (visit_reference_op_load): Record the
        referece into the hashtable twice in case last_vuse is
        different from the original vuse on the stmt.

        * gcc.dg/tree-ssa/ssa-fre-95.c: New testcase.
---
 gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-95.c | 25 ++++++++++++++++++++++
 gcc/tree-ssa-sccvn.c                       | 17 +++++++++++----
 2 files changed, 38 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-95.c

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-95.c 
b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-95.c
new file mode 100644
index 00000000000..b0936be5e77
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-95.c
@@ -0,0 +1,25 @@
+/* PR100112 and dups.  */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-fre1-details -fdump-tree-optimized" } */
+
+int *c, *b;
+void foo()
+{
+  int *tem = b;
+  *tem = 0;
+  int *footem = c;
+  c = footem;
+}
+
+void bar()
+{
+  int *tem = b;
+  int *bartem = c;
+  *tem = 0;
+  c = bartem;
+}
+
+/* We should elide the redundant store in foo, in bar it is not redundant since
+   the *tem = 0 store might alias.  */
+/* { dg-final { scan-tree-dump "Deleted redundant store c = footem" "fre1" } } 
*/ 
+/* { dg-final { scan-tree-dump "c = bartem" "optimized" } } */ 
diff --git a/gcc/tree-ssa-sccvn.c b/gcc/tree-ssa-sccvn.c
index e8b1c39184d..416a5252144 100644
--- a/gcc/tree-ssa-sccvn.c
+++ b/gcc/tree-ssa-sccvn.c
@@ -5125,13 +5125,12 @@ static bool
 visit_reference_op_load (tree lhs, tree op, gimple *stmt)
 {
   bool changed = false;
-  tree last_vuse;
   tree result;
   vn_reference_t res;
 
-  last_vuse = gimple_vuse (stmt);
-  result = vn_reference_lookup (op, gimple_vuse (stmt),
-                               default_vn_walk_kind, &res, true, &last_vuse);
+  tree vuse = gimple_vuse (stmt);
+  tree last_vuse = vuse;
+  result = vn_reference_lookup (op, vuse, default_vn_walk_kind, &res, true, 
&last_vuse);
 
   /* We handle type-punning through unions by value-numbering based
      on offset and size of the access.  Be prepared to handle a
@@ -5174,6 +5173,16 @@ visit_reference_op_load (tree lhs, tree op, gimple *stmt)
     {
       changed = set_ssa_val_to (lhs, lhs);
       vn_reference_insert (op, lhs, last_vuse, NULL_TREE);
+      if (vuse && SSA_VAL (last_vuse) != SSA_VAL (vuse))
+       {
+         if (dump_file && (dump_flags & TDF_DETAILS))
+           {
+             fprintf (dump_file, "Using extra use virtual operand ");
+             print_generic_expr (dump_file, last_vuse);
+             fprintf (dump_file, "\n");
+           }
+         vn_reference_insert (op, lhs, vuse, NULL_TREE);
+       }
     }
 
   return changed;
-- 
2.31.1